Golf ball cores, whether single-piece cores or multi-layered cores, can be manufactured by a variety of conventional processes, including compression molding and injection molding. The golf balls which incorporate the cores can be divided into several general types or groups: two-piece balls, wound (or three-piece) balls, and balls with multi-layered cores and/or multi-layered covers.
Two-piece golf balls are generally most popular with the average recreational golfer because they provide a very durable ball while also providing maximum distance. These balls contain a solid core, typically made of crosslinked polybutadiene, and a cover, typically made of a tough, cut-proof ionomer resin, such as SURLYN.RTM. (DuPont). The combination of the core and cover materials provides a "hard" ball that is virtually indestructible by golfers. Further, such a combination imparts a high initial velocity to the ball, which results in improved distance. Because these materials are very rigid, two-piece balls have a hard "feel" when struck with a club. Likewise, due to their hardness, these balls have a relatively low spin rate, which provides greater distance, but also makes them difficult to control, particularly on shorter approach shots. Because of their simple, two-piece structure, these balls are relatively easy to manufacture using well-known, conventional methods.
Three-piece balls, or wound balls, are the preferred balls of more advanced players, due to their spin and feel characteristics. Wound balls typically have either a solid rubber or liquid center core around which many yards of a stretched elastic thread or yarn are wound. The wound core is then covered with a durable cover material, such as SURLYN.RTM. or a similar material, or a softer cover such as Balata or polyurethane. Wound balls are generally softer and provide more spin, which enables a skilled golfer to have more control over the ball's flight and final position. However, wound, higher-spinning balls typically provide a shorter distance compared to two-piece balls. Moreover, as a result of their more complex structure, wound balls generally require a longer time to manufacture and are more expensive to produce than two-piece balls.
A number of patents are directed towards modifying the properties of conventional non-wound balls by altering the typical single layer core and single layer cover construction to provide a multi-layered ball. These patents are generally directed towards improving a variety of golf ball characteristics by altering the physical and chemical properties of the different layers.
For example, several patents are directed towards improving the carry distance of the ball. U.S. Pat. No. 4,863,167 relates to a multi-layered golf ball having improved rebound characteristics in order to increase its flight distance. The improved characteristics are provided by including a filler in an outer layer so as to impart a higher specific gravity to the outer layer than that of an inner layer. The outer layer is formed using conventional compression or injection molding methods.
Similarly, U.S. Pat. No. 5,184,828 relates to a multi-layered golf ball having improved rebound characteristics and carry distance while maintaining an adequate spin rate. These characteristics are obtained by controlling the size of core inner and outer layers as well as their specific gravity and hardness. While the reference is explicit about the ball characteristics, it does not teach in detail how the outer layer is formed, but indicates only that the outer layer is compression or injection molded around the center.
Additionally, there are a number of patents directed towards improving the spin, click and feel of solid balls while maintaining the distance provided by the solid construction. For example, U.S. Pat. No. 5,072,944 discloses a multi-layered golf ball having a center and outer layer having particular hardness values. With regard to the manufacturing method, however, this reference only discloses that the outer layer is "concentrically molded" on the center core. Other patents, such as U.S. Pat. Nos. 4,848,770, 5,253,871 and 5,688,191 modify the properties of the ball by adjusting factors such as the number of layers, their thickness, and their chemical composition. While these golf balls offer improved characteristics, their manufacturing methods are conventional.
Several patents are directed to improved manufacturing processes. For example, U.S. Pat. No. 5,006,297 discloses a method in which partially cured polyurethane half-shells are formed, enclosed around a center in a fixed-pin mold, intermediately cured, then compression molded to form a finished golf ball. This method, however, is not generally applicable to forming multi-layered cores of different formulations.
U.S. Pat. No. 5,314,187 discloses a method in which an inner cover layer is molded around a core, either by injection molding around the core, or by injection molding two half shells and compression molding the half shells around the core. U.S. Pat. No. 5,586,950 discloses a similar method. However, both of these patents concern inner cover layers, which are thermoplastic materials and which are able to be remolded as a result.
What is seen from these various references is that much attention has been directed to improving the physical properties of golf balls by adjusting such factors as the number and size of layers, the chemical composition of the different layers, the degree of chemical crosslinking and the different densities of the layers. Although multi-layered cores can provide new and improved characteristics to the resulting golf balls, they introduce new demands on the manufacturing process. However, relatively little attention has been directed to improving the manufacturing processes which are used. Those references which do teach manufacturing processes fail to teach processes which can improve the properties of multi-layered cores using a variety of core formulations.
Multiple outer layers are conventionally formed around a golf ball center by a variety of methods, including compression molding or injection molding a spherical shell layer around the center. Each of these manufacturing methods suffers from several disadvantages. Injection molding of the shells around the golf ball center is expensive, because of higher capital costs associated with the more complex manufacturing process, and further can introduce defects into the core due to the penetration by the pins. Moreover, injection molding does not generally work well with thermoset materials. Alternatively, the method of forming hemispherical shells first and then compression molding them around a golf ball center avoids pin defects. This method generally works well with thermoplastic materials. However, the method of forming shells and compression molding them over a core is not generally contemplated with thermoset materials, which lack sufficient rigidity for forming before being cured, and which cannot be reformed after being cured. Thus, the multi-layered cores formed today generally have poor concentricity between the center and outer layers, which adversely affects playing characteristics.
Thus, there remains a need for improved methods of forming multi-layered cores utilizing a variety of core formulations and producing golf ball cores with improved centering characteristics.